1. Field of the Invention
The present invention relates to a solid imaging device and a method of manufacturing the same, and particularly relates to a frame transfer or a full frame transfer-type solid imaging device, which is provided with the improved sensitivity and resolution without reducing the transfer efficiency or the transfer charge.
2. Background Art
When CCD type solid imaging devices are classified by their operation mode, there are two systems: one is an interline transfer system and another one is a frame transfer system (or a full-frame transfer system). In the solid imaging device operated by the interline transfer system, each pixel is constructed by a PN junction, and light is incident on the N-type region through the insulating film formed on the region. A vertical CCD resistor is formed adjacent to each pixel in sequence, and the signal charge accumulated on the light receiving portion is transferred to the vertical CCD resistor. The content of the vertical resistor is transferred and output to the horizontal CCD resistor. In contrast, in the frame transfer-type solid imaging device, the CCD is divided into the light receiving portion and the charge storing portion, the signal charge accumulated in the light receiving portion is transferred to the charge storing portion, and the signal charge stored in the charge storing portion is output to the horizontal CCD resistor. In the case of the frame transfer, the transfer of signals is carried out during unoccupied time, and the light receiving portion stores the next signal charge, during the reading period. Therefore, in some cases, as the light receiving portion in the frame transfer type solid imaging device, the other type of CCD is used in which light is admitted through the transparent electrode forming a transfer gate and the photoelectric conversion is conducted at the PN junction below the transfer gate.
The solid imaging device of the full-frame transfer system comprises a light receiving cell array and a horizontal transfer portion, and when the accumulated charge is transferred to the horizontal transfer portion it is necessary to intercept incident light by means of a shutter such as a mechanical shutter. Since the frame transfer type solid imaging device comprises the light receiving cell array portion, the storing portion and a horizontal transfer portion, the charge accumulated in the storing portion is collectively transferred to the storing portion at high speed. Furthermore, since the storing portion is shaded, transfer of the image information stored in the storing portion can be completed by the time the next image information is accumulated in the light receiving portion, it is not necessary to provide a shutter, such as a mechanical shutter, for intercepting the incident light.
FIG. 7 is a cross-sectional diagram showing a structural example of conventional frame transfer type solid imaging devices. The diagram shown in FIG. 7 illustrates the cross-section of the solid imaging device along the longitudinal direction of the transparent electrode. This solid imaging device comprises, on the P-type silicon substrate, an N-type region 52 corresponding to a photoelectric conversion region 51, and a P+-type region 54 corresponding to a channel stop region 53, which separates the adjacent photoelectric regions from each other. Furthermore, a transparent film 56 is formed through an insulating layer 55 on the substrate 50, and a flattening layer 57 is formed on the transparent film 56.
A manufacturing process of the solid imaging device having the above construction will be described. First, as shown in FIG. 8(a), the N-type region 52 and the P+-type region 54 are formed on the P-type silicon substrate 50, the insulating layer 55 and a polycrystalline silicon film are formed in sequence, and an elongated transparent electrode 56 is then formed by patterning the polycrystalline silicon. Subsequently, as shown in FIG. 8(b), a flattening layer 57 made of silicon oxide is formed so as to cover the transparent electrode 56.
In this solid imaging device, light is incident to the P-type silicon substrate through the flattening layer, the transparent electrode 57, and the insulating layer 55, and after photoelectric conversion is carried out in the N-type region 51 of the photoelectric conversion region 51, the signal charge is stored. The stored signal charges are transferred sequentially by applying pulses to a plurality of transparent electrodes 56.
However, several problems have been encountered in the conventional solid imaging device: the sensitivity is reduced when the transparent film is thick, and the transfer efficiency and the quantity of the transfer charge are reduced when the transparent film is thin. The resolution of the conventional imaging device is also not satisfactory.
The problems are caused by the following factors. In the conventional imaging device, the transparent electrode made of silicon and the like is formed at an uniform thickness, and one of the measures to reduce the wiring resistance around the transparent electrode is to increase the thickness of the transparent film. However, when the thickness of the transparent film is increased, a part of the light incident to the transparent film is diffused, and the transparency of the transparent electrode decreases so that the sensitivity of the imaging device is reduced. If the thin transparent electrode is used in order to increase the sensitivity, the wiring resistance increases, which results in causing a problem of the pulse rounding, and in decreasing the transfer efficiency and the transfer charge quantity. Furthermore, when the structure shown in FIG. 7 is considered, light incident to the periphery of the channel stop region through the transparent electrode is distributed to both of the photoelectric conversion regions of the channel stop region to cause photoelectric conversion, which results in the reduction of the resolution of the imaging device.